The invention relates to a condenser-monochromator arrangement for X-ray radiation in accordance with the features in the preamble of claim 1.
In X-ray microscopy, substantial progress has been made over recent years in the wavelength region of approximately 0.2-5 nm. X-ray microscopes have been developed which are being operated using brilliant X-ray sources. Said X-ray sources include electron storage rings whose deflecting magnets and undulators are sources of intensive X-ray radiation; there have not so far been other X-ray sources of comparable brilliance. To date only the X-ray radiation generated by deflecting magnets has been used for transmitting X-ray microscopes.
At present, only microscope zone plates are used as highly resolving objectives in X-ray microscopes. Microscope zone plates are rotational symmetrical circular transmission gratings with grating constants which decrease outward, and typically have diameters of up to 0.1 mm and a few hundred zones. The numerical aperture of a zone plate is determined very generally by the diffraction angle at which the outer, and thus finest zones diffract vertically incident X-ray beams. The achievable spatial resolution of a zone plate is determined by its numerical aperture. Over recent years, it has been possible for the numerical aperture of the X-ray objectives used to be substantially increased, with the result that their resolution has improved. This trend to higher resolution will continue.
Object illumination of hollow conical shape is generally required for X-ray microscopes which use zone plates as X-ray objectives. Otherwise, the radiation from the zero and the first diffraction orders of the condenser zone plate would also overlap the image at its center. The reason for this is that the overwhelming proportion of the radiation which falls onto the object in a fashion parallel or virtually parallel to the optical axis penetrates said object and the following microscope zone plate (the diffracting X-ray objective) without being diffracted and is seen as a general diffuse background in the direction straight ahead, that is to say in the center of the image field. For this reason, all transmitting X-ray microscopes use annular condensers, and the useful region, not diffusely overexposed region, of the image field becomes larger the larger the inner, radiation-free solid angle region of the condenser.
It is known from the theory of microscopy that the numerical aperture of the illuminating condenser of a transmitted-light microscope should always be approximately matched to the numerical aperture of the microscope objective, in order also to obtain an incoherent object illumination from incoherently radiating light sources, and thus to obtain a virtually linear relationship between object intensity and image intensity. If the aperture of the condenser, by contrast, is less than that of the microscope objective, a partially coherent image is present, and the linear transformation between object intensity and image intensity is lost for the important high spatial frequencies, which determine the resolution of the microscope.
To date, "large-area" annular zone plates have been used as condensers for X-ray radiation. (A. Schlachetzki, K. Dorenwendt: Quantitative Mikroskopie und Mikrostrukturierung [Quantitative microscopy and microstructuring], block seminar from Sep. 13 to 14, 1995, Physikalisch Technische Bundesanstalt, Technische Universitat Braunschweig [Federal Engineering Institute, Braunschweig Technical University], published: PTB-Opt-50, Braunschweig, March 1996, pages 98-116, B. Niemann et al., "X-Ray Microscopy" (see FIG. 3); P. C. Cheng, G. J. Jan: X-ray Microscopy, Springerverlag Berlin Heidelberg 1987, pages 32-38, W. Meyer-Ilse et al., "Status of X-ray Microscopy Experiments at the BESSY Laboratory" (see FIG. 3.1)). They focus the X-ray radiation onto the object to be investigated using the X-ray microscope. The size of such a "condenser zone plate" is matched to the beam diameter, which is typically up to 1 cm at the end of the beam tube of a deflecting magnet of an electron storage ring. Since the condenser zone plate is annular, it captures approximately 3/4 of the radiation situated in said beam diameter. Since the focal length of a zone plate is the reciprocal of the wavelength used, such a condenser zone plate acts together with a small so-called monochromator pinhole diaphragm, which is situated in the object plane about the object, simultaneously as a linear monochromator (Optics Communication 12, pages 160-163, 1974, "Soft X-Ray Imaging Zone Plates with Large Zone Numbers for Microscopic and Spectroscopic Applications", Niemann, Rudolph, Schmahl). Only a narrow spectral region of the incident polychromatic radiation of an electron storage ring is focused into the pinhole diaphragm and used to illuminate the object.
The spectral resolution of such a linear monochromator is R=D/2d, if D and d are the diameter of the condenser zone plate and monochromator pinhole diaphragm and if the condenser zone plate images the source region of the X-ray radiation in a strongly reduced fashion. However, the relationship holds only if the image of the source--it being the so-called "critical illumination" which is concerned here--is not larger than the diameter d of the pinhole diaphragm. If R is at least as large as the zone number n of the microscope zone plate of the X-ray microscope, the chromatic aberration of the microscope zone plate is negligible and worsens the quality of the X-ray image only unsubstantially. In order to satisfy the requirement placed on the spectral resolution R, use is always made of a condenser zone plate of not too small a diameter D, with the result that the permitted diameter d of the monochromator pinhole diaphragm is larger than the image of the source.
Since, for practical reasons, the location of an X-ray microscope can never be brought near the source of the X-ray radiation of an electron storage ring and the separation is typically at least 15 m, the area illuminated by the beam can also not undershoot specific values. Consequently, the diameter D of a condenser zone plate capturing as much X-ray radiation as possible should also not undershoot said values. If the numerical aperture of the condenser zone plate is now increased for these conditions of use, there is necessarily a decrease in the focal length of the condenser zone plate. As a result, there is a reduction in the image scale with which the source is imaged into the object plane, and the diameter of the illuminated object region drops (in practice to a diameter of a few .mu.m), and this is disadvantageous. Only by means of other measures--for example, scanning parallel movements of the condenser and monochromator pinhole diaphragm--is it then possible to ensure that a relatively large object region is homogeneously illuminated. In addition, during the movement, the monochromator diaphragm and condenser zone plate must remain exactly adjusted relative to one another.
Condenser zone plates are normally used at the first diffraction order, at which all condenser zone plates implemented to date have their highest diffraction efficiency. It is difficult in this case to achieve the previously required matching of the numerical aperture of the condenser zone plate to that of the microscope zone plate without coming across new difficulties. In order to realize the matching, the condenser zone plate must have the same fine zones on the outside as does the microscope zone plate itself. The microscope zone plates built with the highest light-gathering power meanwhile have zone widths of only 19 nm (corresponding to a 38 nm period of the zone structures). Zone plates with such fine zone structures can so far be produced only using methods of electron beam lithography, in which the zones are produced successively. Holographic methods, which produce the pattern of a zone plate in one step in a "parallel" fashion and thus in a short time are ruled out, since a suitably shortwave UV holography does not exist. Consequently, it would also be possible to produce condenser zone plates with matched numerical apertures only using methods of electron beam lithography, and this must be described as a serial, and thus slow method. Because of their necessarily large diameter, however, such condenser zone plates typically have several 10,000 zones. The write times with an electron beam lithography system are then of the order of magnitude of weeks, which is unrealistic in practice, for which reason condenser zone plates have to date not been produced using methods of electron beam lithography.
Condenser-monochromator arrangements of even higher light-gathering power are required for dark-field X-ray microscopy (if an absorbing ring, which is to be adjusted very precisely, is not placed in the rear focal plane of the microscope objective). The periods of the zone structures of suitable condenser zone plates would, in turn, need to be less than 38 nm.
A condenser-monochromator arrangement which as far as possible delivers all the X-ray light made available by the beam tube into an annular hollow conical aperture of large aperture angle relative to the object is advantageous for phase-contrast X-ray microscopy.
In order to increase the resolution of the X-ray microscopes, work is presently being carried out on developing microscope zone plates which have a minimum zone width of still only 10 nm. This increases the apertures of the microscope zone plates and, consequently, the required numerical apertures of the condensers, in order to ensure an incoherent object illumination, and the already mentioned difficulties are compounded further.
Electron storage rings which make X-ray radiation available from undulators are under construction, and partly finished, across the globe. Said undulators supply an approximately 10 to 100 times higher X-ray flux, which can be fully used for X-ray microscopy. Moreover, the X-ray radiation is much more effective collimated; the beam at the end of a beam tube typically has a diameter of only 1-2 mm at the location of a microscope, and the "large" condenser zone plates which have been used to date and whose aperture has not been matched can no longer be fully illuminated. In order for condenser zone plates to render the radiation sufficiently monochromatic, it would then be necessary either to use arrangements having the disadvantages already discussed above--smaller condenser zone plates with shorter focal lengths and correspondingly smaller monochromator pinhole diaphragms--or large condenser zone plates must be illuminated in off-axis fashion, that is to say in an edge region. However, such off-axis arrangements illuminate the object obliquely, and this leads to an asymmetric optical transfer function of the microscope, and the images produced thereby can be evaluated only with difficulty. Another avenue which has already been explored consists in suitably expanding the beam by means of an additional zone plate upstream of the condenser. However, this has the disadvantage that further light loss occurs at said additional diffracting element--the diffraction efficiency of zone plates is in the region of only 10% to 20%--and, in addition, there is then a total of three zone plates present in the microscope which, because their focal lengths depend on wavelength, can be adjusted to one another exactly with much more difficulty than two zone plates. Moreover, in the two last-mentioned cases, as well, matching the apertures can disadvantageously be achieved only by matching the smallest zone widths of the condenser zone plate to those of the microscope zone plate.